The existence of a non-zero permanent electric dipole moment
(EDM) of an elementary particle or a composite system of
particles would violate
time reversal symmetry (T), as well as
the combined charge conjugation and parity symmetry (CP),
due to the CPT theorem [1]}.
One of the principal motivations behind
the experimental searches of EDMs
is to shed light on the observed
matter-antimatter asymmetry in the Universe, which in turn
is linked to an asymmetry in the Big Bang baryon-antibaryon
production.
The standard model (SM) of elementary particles cannot explain
the matter-antimatter asymmetry in the Universe,
as SM predicts sources of CP violation
(and of EDMs) several orders of magnitude weaker
than those needed to account for the observed baryon numbers.
This leads to proliferation of the extensions to the
standard model.
Some of these extensions predict larger EDMs, sometimes within
the reach of current experiments.
The experimental searches have not yet detected a non-zero EDM,
but they continue to improve the limits on EDMs of individual
elementary particles, as well as limits on CP-violating
interactions, usually parametrized by the
interaction constants
\(C_{T}\) , and
\(C_{P}\) 
(see section  for details
and the Table II in the reference [2]} for a summary).
These limits
constrain the theoretical extensions of the standard model of elementary
particles.
In recent years these constraints have been set by the measurements
of EDMs of
neutrons [3]},
electrons in a paramagnetic atom
(a thallium atom experiment [4]}),
electrons in a diamagnetic atom
(mercury atom [2]}),
and in TlF and YbF molecules [6]}.
The search for EDMs is not restricted to the above
species, though — see
e.g. [7]}.
